BERKELEY, Calif. — THE global warming crowd has a problem. For all of its warnings, and despite a steady escalation of greenhouse gas emissions into the atmosphere, the planet's average surface temperature has remained pretty much the same for the last 15 years.
As you might guess, skeptics of warming were in full attack mode as the Intergovernmental Panel on Climate Change gathered in Sweden this week to approve its latest findings about our warming planet. The skeptics argue that this recent plateau illustrates what they always knew — that complex global climate models have no predictive capability and that, therefore, there is no proof of global warming, human-caused or not.
Greenhouse theorists appear to be on the defensive as they offer different explanations for the letup — that deep ocean water may be draining some warmth from the atmosphere, that increases in high-altitude water vapor may be responsible or that numerous small volcanic eruptions are the cause.
My analysis is different. Berkeley Earth, a team of scientists I helped establish, found that the average land temperature had risen 1.5 degrees Celsius over the past 250 years. Solar variability didn't match the pattern; greenhouse gases did.
As for the recent plateau, I predicted it, back in 2004. Well, not exactly. In an essay published online then at MIT Technology Review, I worried that the famous "hockey stick" graph plotted by three American climatologists in the late 1990s portrayed the global warming curve with too much certainty and inappropriate simplicity. The graph shows a long, relatively unwavering line of temperatures across the last millennium (the stick), followed by a sharp, upward turn of warming over the last century (the blade). The upward turn implied that greenhouse gases had become so dominant that future temperatures would rise well above their variability and closely track carbon dioxide levels in the atmosphere.
I knew that wasn't the case. The planet warmed by 0.6 degrees over the prior 50 years, but occasional, unexplained temperature fluctuations of as much as 0.3 degrees countered the rise at times and resulted in apparent pauses. Some of the fluctuations might have been caused by shifting ocean currents related to the Gulf Stream and El Niño — the episodic appearance of unusually warm ocean temperatures along the west coast of South America. Here's what I wrote in 2004:
"Suppose... future measurements in the years 2005-2015 show a clear and distinct global cooling trend. (It could happen.) If we mistakenly took the hockey stick seriously — that is, if we believed that natural fluctuations in climate are small — then we might conclude (mistakenly) that the cooling could not be just a random fluctuation on top of a long-term warming trend, since according to the hockey stick, such fluctuations are negligible. And that might lead in turn to the mistaken conclusion that global warming predictions are a lot of hooey. If, on the other hand, we reject the hockey stick, and recognize that natural fluctuations can be large, then we will not be misled by a few years of random cooling."
O.K., I didn't actually predict a pause in the warming but a possible period of cooling. But that's close enough. We are now in that pause, and too many people are taking it too seriously, not just the skeptics and the media but even the greenhouse-warming advocates.
We don't fully understand past variations, but there is a theorem in science: if it happens, it must be possible. The frequent rises and falls, virtually a stair-step pattern, are part of the historic record, and there is no expectation that they will stop, whatever their cause. A realistic prediction simply includes a similar variability as an unexplained component.
Of course, there are scientists who thought they had explained the variability. Previous pauses in temperature rise in 1982 and 1991 were attributed to the ash and sulfur aerosols spewed into the atmosphere by the volcanic eruptions of El Chichón in Mexico and Pinatubo in the Philippines, respectively. I never found those attributions compelling; in particular, the eruption of El Chichón was too small to account for the stall in warming that was attributed to it. I suspect it was more likely that the variations were the result of chaotic changes in ocean currents.
Because of the instability of ocean flow, the best evidence of a changing climate may be the land temperature record. It is full of fits and starts that make the upward trend vanish for short periods. Regardless of whether we understand them, there is no reason to expect them to stop. The best statistical test of an observation is to see if it has happened naturally in the past.
Most of us hope that global warming actually has stopped. (Not everyone; some argue that the warming is good.) Perhaps the negative feedback of cloud cover has kicked in, dampening global warming, or the ocean absorption of atmospheric heat is playing a new and more decisive role.
Alas, I think such optimism is premature. The current pause is consistent with numerous prior pauses. When walking up stairs in a tall building, it is a mistake to interpret a landing as the end of the climb. The slow rate of warming of the recent past is consistent with the kind of variability that some of us predicted nearly a decade ago.
Richard A. Muller is a professor of physics at the University of California, Berkeley, and the author of "Energy for Future Presidents."
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